We plan to recruit 200 healthy, normal children and adolescents in this study, age ranging from 3 to 15 years. All of them will be invited to undergo an overnight polysomnography for at least 7h in Beijing Children's Sleep Center, meanwhile the parameters about sleep including EEG, electromyography, electrooculography, ECG, pulse oximetry arterial oxygen saturation (SpO2), chest wall and abdomen motion, oral and nasal airflow, and end-tidal PCO2 (PETCO2) will be recorded.